1. Field of the Invention
The present invention relates to a process for heat treating of high denier filaments of a thermotropic liquid crystalline polymer. Specifically, the present invention provides a process for heat-treating high denier filaments made of a variety of thermotropic liquid crystalline wholly aromatic polyesters and polyesteramides directly on the bobbin on which the filaments are wound.
2. Description of the Prior Art
Thermotropic liquid crystalline polymers (LCPs) are an important class of polymers which are generally wholly aromatic molecules containing a variety of heteroatom linkages including ester and/or esteramide linkages. Upon heating to sufficiently high temperature, LCPs melt to form a liquid crystalline melt phase (often referred to as "anisotropic phase") rather than an isotropic melt phase. Generally, LCPs consist of linear ("rigid rod") molecules that can line up to yield the desired liquid crystalline order. As a result, LCPs feature low melt viscosity and thus improved performance and processabilities.
Because LCPs orient to form "rigid rod" linear molecules, LCPs exhibit extremely high mechanical properties. Thus, it is well known in the art that LCPs can be formed into shaped articles such as films, rods, pipes, fibers and various other molded articles. In addition, it is also known in the art that LCPs, particularly in the fiber form, exhibit exceptionally high mechanical properties after a heat treatment process. However, all of the known methods in the art describe formation of only the low denier fibers, e.g., of about 10 deniers per filament (dpf), which exhibit high mechanical properties in their as-spun as well as heat-treated forms.
It is an object of the present invention to provide a process for forming uniformly oriented high denier LCP filaments. High denier filament means a filament of higher than 50 dpf,
It is also an object of the present invention to provide a process for forming high denier LCP filaments of higher than 50 dpf, which filament exhibit enhanced mechanical, thermal and chemical resistance properties in the as-spun as well as heat-treated form.
It is further an object of the present invention to provide a process for forming high denier LCP filaments, which filament exhibit properties comparable to those of low denier LCP filaments (i.e., filaments of less than 10 dpf) in their as-spun as well as heat treated states.
It is also an object of the present invention to provide high denier LCP filaments of higher than 50 dpf having properties comparable to those of low denier LCP filaments of less than 10 dpf.
Finally, it is an object of the present invention to provide a cost-effective, industrially economic way to heat-treat the high denier filaments of this invention directly on the bobbin so as to produce high denier filaments of superior mechanical and physical properties.
It is highly desired to form uniformly oriented high denier LCP filaments which exhibit enhanced mechanical thermal and chemical resistance properties in the as-spun as well as heat-treated form. For example, such high denier LCP filaments can replace steel wires in steel belted tires. Furthermore, since LCP filaments are of substantially lower density when compared with steel wires, LCP filaments are expected to feature much superior properties compared to those exhibited by steel wires. It is further obvious from the following prior art that there is a real need for high denier LCP filaments that exhibit enhanced mechanical, thermal, and chemical resistance properties.
3. Prior Art
The following references are disclosed as background prior art.
U.S. Pat. No. 4,183,895 describes a process for treating anisotropic melt forming polymeric products. It is claimed that a process of heat treatment obtained fibers having enhanced mechanical properties, and the fiber tenacity was increased by at least 50% and to at least 10 grams per denier.
U.S. Pat. No. 4,468,364 describes a process for extruding thermotropic liquid crystalline polymers (LCPs). It is claimed that extrusion of an LCP through a die orifice having an L/D ratio of less than 2 (preferably 0), and at a draw-down ratio of less than 4 (preferably 1), one can obtain filaments featuring high mechanical properties.
U.S. Pat. No. 4,910,057 describes a highly elongated member of substantially uniform cross-sectional configuration, which is capable of improved service as a stiffening support in an optical fiber cable.
U.S. Pat. No. 5,246,776 describes an aramid monofilament and method of making the same.
U.S. Pat. No. 5,427,165 describes a reinforcement assemblage formed at least in part of continuous monofilaments of liquid crystal organic polymer(s). The polymers used therein are primarily aramids.
Japanese laid open Pat. No. 4-333616 describes a method of manufacturing filaments of 50 to 2000 dpf from molten liquid crystalline polymers. The heat-treated mechanical properties of these filaments were significantly inferior than the properties reported for the corresponding lower denier filaments of 5 to 10 dpf,
J. Rheology 1992, Vol. 36 (p. 1057-1078) reports a study of the rheology and orientation behavior of a thermotropic liquid crystalline polyester using capillary dies of different aspect ratios.
J. Appl. Polym. Sci. 1995, Vol. 55 (p. 1489-1493) reports orientation distribution in extruded rods of a thermotropic liquid crystalline polyesters. The orientation function increases with increasing apparent shear rate from 166 to 270 sec.sup.-1, but decreases with increasing apparent shear rate from 566 to 780 sec.sup.-1.
All of the references described herein are incorporated herein by reference in their entirety.